In recent years, as a radical solution of energetic and/or environmental problems, and, further, as a central energy conversion system in the future age of hydrogen energy, fuel cell technique has drawn attention. Especially, polymer electrolyte fuel cells (PEFC) are tried to be applied as power sources for electric automobiles, power sources for portable instruments, and, further, applied to domestically stationary power source apparatuses utilizing electricity and heat at the same time, for the reason that miniaturization and lightening are possible, etc.
A polymer electrolyte fuel cell is generally composed as follows. First, on both sides of a polymer electrolyte membrane having ion conductivity, catalyst layers comprising a platinum group metal catalyst supported on carbon powder and an ion-conducting binder comprising a polymer electrolyte are formed, respectively. On the outsides of the catalyst layers, gas diffusion layers as porous materials through which fuel gas and oxidant gas can pass are formed, respectively. As the gas diffusion layers, carbon paper, carbon cloth, etc. are used. An integrated combination of the catalyst layer and the gas diffusion layer is called a gas diffusion electrode, and a structure wherein a pair of gas diffusion electrodes are bonded to the electrolyte membrane so that the catalyst layers can face to the electrolyte membrane, respectively, is called a membrane electrode assembly (MEA). On both sides of the membrane electrode assembly, separators having electric conductivity and gastightness are placed. Gas passages supplying the fuel gas or oxidant gas (e.g., air) onto the electrode surfaces are formed, respectively, at the contact parts of the membrane electrode assembly and the separators or inside the separators. Power generation is started by supplying a fuel gas such as hydrogen or methanol to one electrode (fuel electrode) and an oxidant gas containing oxygen such as air to the other electrode (oxygen electrode). Namely, the fuel gas is ionized at the fuel electrode to form protons and electrons, the protons pass through the electrolyte membrane and transferred to the oxygen electrode, the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode, and they react with the oxidant gas to form water. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
For practical implementation and spread of polymer electrolyte fuel cells, as to the aspect of performance, in addition to high power generation performance, it is important that they can be operated stably for a long time. In polymer electrolyte fuel cells, particularly polymer electrolyte fuel cells using methanol as a fuel, the structure of the electrolyte membrane, particularly the structure of ion-conducting channels formed by aggregation of sulfonic acid groups or the like as an ion-conducting group is liable to change, and, thus, power generation characteristics are also liable to change. Therefore, an electrolyte membrane is desired which, on the one hand, has high power generation performance, and, on the other hand, is not easily influenced by methanol, for example, an electrolyte membrane which, in addition to high power generation performance, has low methanol permeability, or an electrolyte membrane wherein, in addition to high power generation performance, change of characteristics, particularly characteristics such as methanol permeability and ion conductivity between before and after power generation, which, in the electrolyte membrane, corresponds to before and after the treatment of immersion in a methanol solution, is small.
In general, a polymer electrolyte fuel cell is not steadily operated, but starting, operation and stop are made repeatedly. Although, during operation, the polymer electrolyte membrane is under a wet state, during stop, lowering of humidity is liable to occur. Therefore, an electrolyte membrane is desired wherein change of dimensions and/or change of dynamic characteristics between under a state of low humidity (under a dry state) and under a wet state are/is small. Further, an electrolyte membrane is desired which is excellent in starting properties so that it could display a certain level of characteristics immediately after the operation circumstance is changed, for example so that stable operation could be made immediately after starting.
As polymer electrolyte membranes for polymer electrolyte fuel cells, Nafion (registered trade mark of Dupont Co., as is the same hereinafter), which is a perfluorocarbonsulfonic acid polymer, is used by reason of being chemically stable. However, Nafion has a disadvantage that methanol is liable to permeate it, and, in polymer electrolyte fuel cells using methanol as a fuel, a phenomenon that methanol permeates the electrolyte membrane from one electrode side to the other electrode side (methanol crossover) is liable to occur, and, therefore, sufficient performance is hard to obtain. Further, since Nafion has a property that change of dynamic characteristics between during a dry state and during a wet state is large, performance tends to be lowered during a long-term power generation test. In addition, since Nafion is a fluoropolymer, consideration to the environment at the time of its synthesis and disposal is necessary, and fluoropolymers are expensive. Therefore, development of novel electrolyte membranes is desired.
Thus, non-perfluorocarbonsulfonic acid polymer electrolyte membranes having small methanol permeability have been studied (Patent documents 1 to 4 and Non-patent documents 1 to 3).
Engineering plastic polymer electrolyte membranes described in Patent documents 1 to 3 and Non-patent document 1 do not readily form ion channels, which is different from the case of perfluorocarbonsulfonic acid polymer electrolyte membranes, and it is possible to reduce methanol permeability. However, they have a defect that the electric resistance of the membrane is relatively high, and when the electric resistance of the membrane is lowered by increasing the amount of ionic groups introduced, it is liable to swell easily. Further, a defect that imperfect bonding to electrodes tends to occur is also known. Therefore, it is the present state of things that engineering plastic polymer electrolyte membranes have not displayed sufficient performance as an electrolyte membrane used in direct methanol fuel cells.
As polymer electrolyte membranes using a non-fluoropolymer as a base, an electrolyte membrane is also proposed wherein the polystyrene block of a block copolymer composed of styrene and a rubber component is sulfonated to make the rubber component function as a matrix and make the polystyrene block function as ion-conducting channels (Non-patent documents 2 and 3 and Patent document 4). For example, in Non-patent document 2, as an inexpensive and mechanically and chemically stable polymer electrolyte membrane is proposed a polymer electrolyte membrane comprising a sulfonated SEBS (SEBS is an abbreviation of a polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer). In Patent document 4, a polymer electrolyte membrane comprising a sulfonated SEBS is also described as an inexpensive and mechanically and chemically stable polymer electrolyte membrane.
However, it is described that these electrolyte membranes are such that their structure is largely changed in a methanol solution (Non-patent document 2). This suggests that when the resulting electrolyte membranes are immersed in a methanol solution, their characteristics such as methanol permeability and ion conductivity are largely changed. As a result of actual tests by us, it was made clear that these polymer electrolyte membranes are such that, between before and after a treatment of immersion in a methanol solution, characteristics such as methanol permeability and ion conductivity are changed largely, and, between during a dry state and during a wet state, dynamic characteristics such as a tensile characteristic are changed largely. By that change of these characteristics is large, it is meant that when such a membrane is used in a fuel cell, it is difficult to operate it stably for a long time.
It is described in Non-patent document 3 that sulfonated polystyrene-b-polyisobutylene-b-polystyrene triblock copolymers (sulfonated SiBuS) also have higher methanol barrier properties compared with perfluorocarbonsulfonic acid polymer electrolyte membranes, but it is the present state of things that electrolyte membranes having satisfactory performance for direct methanol fuel cells have not yet been obtained.    Patent document 1: JP-A-2003-288916    Patent document 2: JP-A-2003-331868    Patent document 3: JP-A-6-93114    Patent document 4: JP-A-10-503788    Non-patent document 1: J. Membrane Science 197 (2003) 231    Non-patent document 2: J. Membrane Science 217 (2003) 227    Non-patent document 3: J. Membrane Science 214 (2003) 245